The present invention concerns an aircraft having a fuselage fuel tank, a method of reducing fuel flammability in a fuselage fuel tank, and an aircraft fuselage for use with such an aircraft or for use in such a method.
The aircraft industry is continuously looking to improve aircraft safety and to make commercial aircraft flight an even safer way of travelling. One area of activity in this field is increasing safety in fuel tanks by reducing in-tank flammability of fuel. In this regard, it should also be noted that Regulations of the US Federal Aviation Administration (FAA), for example FAR 26.33, set out certain criteria that certain aircraft types will need to comply with.
One method of reducing in-tank fuel flammability is to incorporate an inerting system in the aircraft, whereby the oxygen content in the ullage (the air space in the fuel tank) is reduced. One such inerting system works by injecting inert gases from bottles stored on the aircraft. Another proposal involves generating a nitrogen rich air supply from engine bleed air and air separation devices.
U.S. Pat. No. 6,343,465 discusses a method of improving safety in fuel tanks by effectively sucking out vapour in the fuel tank, or air around the fuel tank, by means of suction created at the engine air intake area. The removed vapour/air is replaced with atmospheric air and/or mechanically cooled cabin-air.
The above proposal and concerning the inerting system requires exchange of gaseous fluids within the interior of the fuel tanks, as does one of the proposals of U.S. Pat. No. 6,343,465. These two proposals appear to be alternatives. It would appear that inerting the ullage is not compatible with the method of U.S. Pat. No. 6,343,465 in which fumes are continuously drawn from the fuel tanks for combustion in the engines, whilst introducing a stream of cooled air with a normal oxygen content. The proposals of U.S. Pat. No. 6,343,465 all rely on using the engine to provide suction, which must necessarily reduce engine efficiency. Vapour/air in and around the fuel tanks if used during the air compression stage of a jet engine would also pollute such compressed air with fuel vapour. Such compressed air could not then be used for cabin air conditioning. Moreover, for the proposals of U.S. Pat. No. 6,343,465 to be practical and safe, there would need to be a sophisticated control system to control air pressure, air flow rates and demand on the engine and/or air conditioning units providing the mechanically cooled air.
It has been shown that, during flight, the temperature of fuel in tanks in the wings of an aircraft is typically significantly lower than the temperature of fuel in tanks in the fuselage. The wings are more exposed to airflow, which at cruising altitude tends to be rather cold. The fuel in the wing tanks is therefore kept relatively cool by means of the cooling effect provided by such cold air passing over the wings. Fuel in tanks in the fuselage tend to be less exposed to the airflow. Parts of the fuel tanks are near the interior of the fuselage, where temperatures need to be warmer in view of passenger comfort. The fuselage may also include systems that generate heat, such as for example air conditioning units used for conditioning cabin air. In some aircraft, one or more air conditioning units are located directly adjacent to the fuselage fuel tanks.
The present invention seeks to provide an alternative or improved method or apparatus for reducing fuel flammability in a fuselage fuel tank. Additionally or alternatively, the present invention seeks to provide an alternative or improved method or apparatus for cooling fuel in a fuselage fuel tank. Additionally or alternatively, the present invention seeks to mitigate one or more of the problems or disadvantages mentioned above.